Free space fiber optic telecommunications devices often require launch optics with different beam waist sizes in orthogonal directions. For example, consider a wavelength selective switch (WSS), which allows certain wavelength components or channels of WDM optical signal to be spatially separated (dispersed) and selectively switched from a first optical fiber to a second optical fiber while letting the other wavelength channels propagate in the first optical fiber, or it allows certain wavelength channels to be switched to a third optical fiber. The resolution (or band edge sharpness) of the switch is determined by the beam waist size in the direction along which the wavelength components are dispersed at the switch focal plane (referred to herein as the dispersion beam waist size), which is equal to the launch optic beam waist size in the dispersion plane times the system magnification. In order to achieve the best resolution (sharpest band edges) the launch optic should produce a small beam waist in the dispersion plane.
On the other hand, the beam waist size in the direction orthogonal to the dispersion direction (the port direction) at the switch focal plane (referred to herein as the port beam waist size) limits the number of ports that the switch can support. A large beam waist has a small angular extent, which allows many ports to be located in a given angular switching range without crosstalk. The port beam waist at the switch focal plane is equal to the beam waist in the port direction of the launch optic times the system magnification. So in order to provide a high port density or total port count, it is desirable that the launch optic produce a large port beam waist.